One of the most important, yet overlooked part of wedding invitations, is postage. Calculating the right amount of postage that will be needed to mail the invitations to your guests, can be quite tricky and can lead to unexpected surprises.
It is also important to consider that postage pricing is constantly changing. The only stamps that retain their value, are the so called “forever stamps”. So a forever stamp that you might have bought 10 years ago for $0.35c, today can be used for a 1oz letter, valued at 0.66c in 2023 (please check current usps prices here
It is very important to always consult a post office representative to help calculating the cost for your wedding invitations to be mailed.

While most of the mail that we receive every day, can be sent with a simple “forever stamp”, on a general basis, a wedding invitation, will require more postage. A forever stamp will be enough to mail a piece of mail that is no more than one ounce, and cannot exceed 11.5 inch x 6.125 inch (and never more than 6.125 inch tall!).
Most of our wedding invitations do respect the size restrictions, but most of the times, once including insert card and response card, end up weighing more than 1 ounce, at which point, will require more postage, based on the weight.
So an invitation weighing at 1.3oz, will require 1 forever stamp + 1 additional-ounce stamp (or alternatively a 2oz stamp)
An invitation weighing 2.4oz will require 1 forever stamp + 2 additional-ounce stamp.
And so on. In any case the weight is the first factor to consider when it comes to calculating postage.

Odd-Size/Nonmachineable Letters
Square letters are considered odd-sized/nonmachineable. Here are some other factors that will result in you paying a postage surcharge:

envelopes that are too rigid
envelopes that don’t bend easily
envelopes that are more than 1/4″ thick
envelopes that have clasps, strings, or other non-standard closures
Typically the postal service will only charge one surcharge even if your mailpiece has two nonmachineable characteristics (such as a square envelope with a clasp). However, be sure to visit your local post office so they can help you determine the exact amount of postage you’ll need to mail your letter.